The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the reduction of image artifacts caused by respiratory motion.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Abdominal and cardiothoracic imaging are important applications of MRI, but image degradation due to respiratory motion remains a major problem because a significant time span is required to acquire the image data. One general strategy for reducing such degradation is to use artifact reduction methods, such as respiratory ordered phase encoding, gradient moment nulling, or view averaging. These all work to some degree. However, a fundamental limitation of all of these methods is that none accounts for the blurring which accompanies the view-to-view motion effects. In the abdomen this motion is principally in the superior-inferior (S/I) direction with a range exceeding 10 mm. This is clearly a problem in imaging sagittal and coronal sections. Additionally, during imaging of transverse sections this motion will cause many structures to move completely in and out of the section during the acquisition.
An alternative strategy is to circumvent respiratory motion effects by using breath-hold image acquisition. This approach has been used in many applications, including angiographic imaging, T.sub.1 -weighted and T.sub.2 -weighted imaging of the abdomen, cine phase contrast imaging, pulmonary imaging, cardiac imaging and imaging of the coronary arteries. Again, these all work to some degree. However, for many of these methods the resolution or signal-to-noise ratio (SNR) could be improved if the acquisition were extended over, or broken into, several periods of suspended respiration. The problem with this is that the degree of chest inflation cannot be accurately reproduced, causing misregistration of the thoracic and abdominal structures from one breath-hold to the next. This results in image artifacts and blurring. Indeed, we have found that the level of patient reproducibility in S/I diaphragmatic position is 8.3 mm, roughly comparable to the thickness of a typical axial section.